How do i know if my girl is faking?

when i have sex how do i know if my girl is faking.. i mean she always be sticking her nails in my back, and telling me she sore... and one day she said she haven't came yet while we be having sex.. and we had sex like 10 times, my penis is about 8inches ....is that bad, am i doing something wrong... how do i make her feel better... cause i wanna please her.